Our goal is the determination of the atomic-level structure of E. coli penicillinase enzyme in the presence of bound penicillin analogs. This work will allow us to suggest ways of designing new penicillin antibiotics which will be less susceptible to breakdown by the beta-lactamase defenses of resistant bacteria. Our x-ray diffraction analysis of the crystalline penicillinase-penicillin complex will show in three-dimensions how various substrates interact with the active site residues during catalytic hydrolysis. With this information chemical modifications to penicillins can be made which will weaken or prevent these interactions. Alternatively, strongly inhibitory or irreversibly binding analogs can be made which will block the active site of the enzyme and prevent the entry of antibiotic molecules. The modified penicillins must have the ability to irreversibly acylate the transpeptidase which is thought to crosslink the peptidoglycan chains in the nascent cell wall network. The active site of this transpeptidase must be very similar to the active site of penicillinase, since both enzymes bind penicillin. To design a penicillin which is penicillinase-resistant and at the same time able to inhibit cell wall growth, it is necessary to determine the very subtle differences which must exist between the active sites of these two enzymes. Collaborative arrangements have now been made for supplying a purified transpeptidase for x-ray analysis.